German Published Examined Application DE-AS 23 60 672 and corresponding U.S. Pat. No. 3,928,748 disclose a prior art heated windshield with antenna. In that conventional vehicle windshield, a system of conductors is provided which simultaneously performs heating and antenna functions. In the windshield, electrical resistance elements are arranged between current supply busses, of which one is located near the upper windshield edge, and the other is located near the lower windshield edge, the heating wires running vertically between the busses. One of the two busses is divided into two parts, the respective adjacent ends of which are capacitively coupled to a receiver. The two parts of the bus, and whatever extensions are attached thereto, thus form a Frequency Modulation (FM) antenna, also known in Germany as a UKW antenna. The specifications of this antenna are derived from theoretical calculation of the lengths of the extensions and subsequent corrections based on empirical measurements.
For some time, there has been discussion of vehicle windshields which are coated over their entire surface with gold or another conductive metal. This metal coating is intended to also serve for windshield heating. For this purpose, busses for heating current supply are arranged at, for example, two opposing windshield edges. Due to the attached metallic layer, none of these busses are adapted to serve as FM antennas. One way nevertheless integrate, into vapor-deposition-coated windshields, which are sensitive in the FM reception bands, as well as in the AM reception bands, if the coating is formed with an uncoated slot having a length approximating a half-wavelength and a width very much smaller, and a minimum spacing from current supply busses on opposing sides of the windshield, and the coating is connected to the receiver.
In the following discussion, the Greek letter lambda or .lambda. is the symbol for the wavelength of a particular radio frequency in the radio band which the antenna is designed to receive, typically an arbitrarily chosen frequency at the middle of the band. It is well known that antennas have a resonant frequency which depends upon their length, and receive best signals at that frequency or some harmonic thereof.